1. Field of the Invention
The present disclosure relates generally to lighting, light fixtures, lamp assemblies and LED lighting, and more particularly, to a programmable LED constant current drive circuit for driving LEDs at constant current and dimming the LEDs using standard, off-the-shelf, wall dimmers.
2. Description of the Related Art
Incandescent light bulbs are used in a large variety of lighting products. Although inexpensive to purchase, incandescent light bulbs have several drawbacks. First, incandescent light bulbs use a relatively large amount of power compared to other lighting products or technologies (e.g., light emitting diode (LED) or compact fluorescent lamp (CFL)) which increase energy costs. Second, incandescent light bulbs have a relatively short life causing repetitive replacement costs. Furthermore, since theses bulbs have a short life, labor costs in commercial applications will subsequently be effected by having maintenance personnel constantly replace the bulbs.
Because of their relatively low efficiency in generating light (95% or more energy is actually turned into heat with only 5% producing light), incandescent bulbs are actually being banned through government regulations at local and federal levels, in several countries around the world. In addition, states such as California have established regulations for new building construction (i.e., Title 24 for commercial and residential buildings) that require minimum levels of lighting energy efficiency which essentially prohibits incandescent bulbs from being used in any large quantity within a building.
Compact fluorescent light (CFL) bulbs, while offering 2-3 times the energy efficiency over incandescent light bulbs, due to their design and light emission properties, can pose limitations in overall efficacy when combined with a light fixture. In addition, CFL bulbs contain mercury (a long term environmental issue), are often slow to warm up to produce rated light levels and are generally not dimmable. CFL bulbs have received mixed reviews from consumers (e.g., aesthetic appearance, light color, noise), though the technology has continued to improve.
A recent trend in the lighting industry is to develop light emitting diode (LED) engines or modules that can be easily adapted to current light fixture products. LED technology offers 3-5 times the energy efficacy of traditional incandescent bulbs and has 25 times the reliability. This offers a potentially large savings in energy consumption in interior and exterior lighting applications. In addition, LEDs produce light which is more “directional”, enabling LED light engine designers to customize the luminous intensity profile for various applications, further enhancing overall light fixture efficacy. While LED technology is generally more expensive, there can be substantial savings in energy cost, bulb replacement and maintenance costs over a multi-year period.
To-date, a number of “socket based” LED products have entered the market to retrofit in place of incandescent bulbs. Some of these products use a large number of lower power LEDs or a fewer number of high power LEDs. Generally, these products have had relatively low light output to replace common light fixture incandescent sources (e.g. 75 W bulb) and poor thermal management properties required to ensure long LED life. In addition, many of these light sources are highly directional and not compatible with many decorative light fixtures (e.g. pendants) detracting from the aesthetic appearance of the fixture and the LED light source.
Thus, a need exists for new LED based light fixtures and LED retrofit lighting products having low power consumption, high light output and effective means for heat dissipation when used within semi-enclosed light fixture products. The retrofit product should be a screw-in replacement for an incandescent or CFL bulb for easy retrofit into the existing installed base of light fixtures in residential and commercial applications. The light engine should convert standard residential and commercial line voltage to a form to drive the LEDs consistently and reliably.
LED systems are increasingly attractive for low voltage applications to replace incandescent and halogen based lamp systems to greatly improve lighting efficiency and reliability. Two examples where LEDs offer great benefits over traditional lighting technologies are in low voltage “track” lighting systems and “under-cabinet” lighting, though there are many other applications, including landscape lighting and cabinet interior lighting. In many of these applications, a range of dimmability is highly desirable to control the lighting intensity in the given installation.
With these systems, a standard magnetic or electronic transformer is typically employed to reduce the input line voltage (e.g., 100-240 VAC) to 12 VAC or 24 VAC. These transformers are attractive because they traditionally support incandescent based low voltage lighting and have a wide range of sources, are produced in high volume and are relatively low in cost. 24 VAC or 12 VAC is used to simplify the fixture, track and wiring system (2 wires) for low cost, flexibility and safety.
In these new LED systems, it is highly desirable to use existing solid-state dimmer technology that has evolved for incandescent lighting over the years to leverage the large installed base of dimmers and their relatively low cost. These “forward phase” dimmers work by varying the “duty cycle” (on/off time) of the input line AC waveform, to effectively control the average voltage over its normal cycle (e.g. 60 Hz). Typical dimmers use thyristors (e.g., Triacs) to switch the power, the timing for which is triggered at the zero crossing of the input power waveform and the dimmer setting (i.e., resistance). Special versions of these dimmers (Magnetic Low Voltage (MLV)) are available to support inductive loads (e.g. magnetic transformers). These dimmers have separate control electronics connected to both the live and neutral wire to better control the Triac switching.
Another type of dimmer, commonplace today, is an Electronic Low Voltage (ELV) dimmer that is based on a transistor design and provides “reverse-phase” or “trailing edge” dimming which is more compatible with the electronic transformer's reactive circuit. This type of dimmer will conduct power at the zero crossing point and then turn off at the adjustable position in the middle of the AC current phase.
The objective of a low voltage dimmable LED system is a good range of dimming (e.g. 10% to 100%), linear operation (i.e., light intensity decreases linearly with dimmer control movement) and no visible flickering effects (that can occur when phase-to-phase (60 hz) performance differences occur). Therefore, a need exists for techniques for dimming LED light engines and LED lighting systems, using standard phase control dimmer technology that is in use today for incandescent lighting and low voltage lighting.
Furthermore, it is difficult to control legacy light fixtures into which users will install these LED light engines. Generally, LED light engines have heat sinks that require cooling via convective means or they can overheat and not perform to their design specifications for light output, efficacy or service life. Therefore, a need exists for a temperature compensated LED driver that can adapt to it's installed environment.